Petrol post hole digger is a kind of engine machine, designed for garden cultivation, tree planting, post hole digging etc. It saves human labor and increases efficiency.
Common petrol post hole diggers only suit for working in even, soft soil ground. If in the process encountering big tree roots or rocks, the drill bit will have a sudden stop which will throw all the inertia and engine torque to operator. Facing this situation, in natural the operator will hold handles firmly and try to control the machine, its much easier to rotate together with the frame and fall down. This is a big safety risk.
Some diggers are built in buffer structure between engine spindles and working drill bits to mitigate the impact force, the results are not satisfactory. Some add brake device in the engine output spindle, relying on friction to reduce the torque transmit. Disadvantage is clutch slipping is very often and heat is made on the friction surface, affect the life time of machines, also not a good idea.